A circle has a sector with area $1\pi$ and central angle $\dfrac{1}{2}\pi$ radian. What is the area of the circle? ${4\pi}$ $\color{#9D38BD}{\dfrac{1}{2}\pi}$ ${1\pi}$
Answer: The ratio between the sector's central angle $\theta$ and $2 \pi$ radians is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{2 \pi} = \dfrac{A_s}{A_c}$ $\dfrac{1}{2}\pi \div 2 \pi = 1\pi \div A_c$ $\dfrac{1}{4} = 1\pi \div A_c$ $A_c \times \dfrac{1}{4} = 1\pi$ $A_c = 1\pi \times 4$ $A_c = 4\pi$